Geothermal system operable between heat recovery and heat storage modes

ABSTRACT

The geothermal system uses an outer and an inner pipe installed in a single borehole. Cool fluids are pumped down through one pipe and returned to the surface through the other pipe. Subterranean heat increases the temperature of the cool fluid and this heat is returned to the surface where the heated fluid is recovered. The fluid with the heat removed is then pumped back down the borehole to be re-heated. Extra heat recovered from the ground surrounding a lower portion of the borehole is stored in the ground and rock formation surrounding an upper portion of the borehole during warmer seasons to optimize the amount of heat stored in the ground for extraction during colder seasons.

This application claims the benefit under 35 U.S.C. 119(e) of U.S.provisional application Ser. No. 62/693,939, filed Jul. 4, 2018.

FIELD OF THE INVENTION

The present invention relates to the recovery of heat from a geothermalsystem including a well in the ground from which heat is extracted, andmore particularly the present invention relates to a geothermal systemwhich is seasonal varied between a heat recovery mode in which heat isdrawn out of the geothermal well and a heat storage mode in which heatis stored in the geothermal well.

BACKGROUND

Below the Earth's surface, temperature increases with depth. The rate oftemperature increase, or geothermal gradient typically ranges between 2and 3° C. per 100 metres of depth. However, in some regions, thegradient can be significantly higher with very hot temperatures foundcloser to the Earth's surface. Current technologies are exploiting someof these near surface hot spots for electrical power generation andheating.

There are two commonly used heat recovery methods for deeper geothermalsystems. The most common practice is where boreholes tap in tounderground hot zones either by drilling a pair of boreholes or a singleborehole that recover heated water from hot rock material at depth. Inthe first case (two well method) one borehole is for injecting coolerwater and the other borehole is for producing the heated water. Thewells are connected at depth usually by an aquifer or by fracturing therock material between the wells. Water is pumped down the injectionwell, through the permeable zone between the wells then pumped back tothe surface in the second well. The other, single well method, is wherean aquifer has sufficient hot water recharge that it can produce heatedwater without the need of a second injection well. These technologiesare used in many countries where subterranean hot zones occur atshallower than normal depths. These existing systems have severaldrawbacks. The cost of drilling the wells is high, and the success rateof properly connecting the two wells at depth can be relatively low.Further, the connected zone between the well pairings is difficult tocontrol as there are fluid losses in to the rock formations. In thiscase a significant amount of pumping is required to counteract the fluidloss. The greatest drawback of the single aquifer pumping well method iswhen there is less than adequate water recharge from the aquifer. Bothsingle and double well methods require large pumps and power to lift thewater from deep underground. Furthermore, the fluid returned to thesurface may contain high levels of noxious contaminants which have to bedisposed of safely. Formational water can also be corrosive whichreduces the life expectancy of the pipes and pumps.

A third, less commonly used method was developed whereby two pipes arecontained within the other in a single well. This coaxial borehole issealed at the base which prevents any interaction between the boreholeand formational fluids and gases. The two pipes act as a long, linearheat exchanger. Water is pumped in to the well between the two pipes andis heated from the adjacent rock formation as the water descends. At thebottom of the borehole the heated water is then redirected to surfacethrough an insulated inner pipe. Once the heated water reaches thesurface, the heat is extracted and the resultant cooled water is thenre-reinjected in to the same well to complete the water circuit. One ofthe advantages over the heated water recovery process is there is noproduction of hazardous fluids and gases. Another advantage is, sincethe closed loop circuit is sealed, the pumping demand is significantlyreduced because there is no hydraulic head to overcome. The disadvantageof this heat exchanger technology is the significantly reduced amount ofheat recovered. Because the descending cool water and the ascendingheated water have to share the same borehole, the volumes of water aresignificantly reduced. Technically it is difficult to achieve sufficientwater residence time for boreholes less than 178 mmm diameter. Further,the higher the flow velocity of the injected water reduces the residencetime which reduces the amount of heat recovered.

There are two key factors that limit the economics of the coaxial welltechnology. The first factor is the high cost and the space restrictionsof the insulated inner pipe. The second factor is the restricted heatrecovery due to the shortened residence time of the fluid during theheat transfer process. Although the heat exchange technology hassignificant environmental benefits, low heat productivity and highcapital costs have prevented this technology from flourishing.

Several patents or patent applications relevant to deep single ormultiple well geothermal energy recovery include the following: (i)Horton, U.S. Pat. No. 5,203,173 for a Device for utilization ofgeothermal energy; and (ii) Montgomery, U.S. Pat. No. 8,708,046 for aClosed Loop Energy Production from Geothermal Reservoirs. The priorpatents or patent applications noted above focus mainly on recoveringhigh temperature geothermal energy for electrical power generation.Therefore, the technology is designed to recover fluids or steam hotenough to drive turbines for power generation. In order to do so, highertemperature heat sources are required. The heat recovery happens in azone near the bottom of the well where temperatures are highest. Pipingsystems above the targeted heat zones are insulated in order to conveythe superheated fluid to the surface so it can be converted to steam forturbine power. Pipes are also coated to prevent deterioration bycorrosive fluids. The Horton and Montgomery patents take a differentapproach and call for the heat exchange within a coaxial, closed loopsystem and within a naturally occurring geothermal gradient.

US Patent Application Publication No. 2011/067399 by Rogers and U.S.Pat. No. 9,121,393 by Schwarck disclose systems which can be operated bypumping fluid down a centre tube so that the fluid rises in heatexchanging relationship with the surrounding wellbore in the annulus.Neither system includes various operating modes according to differentseasonal temperatures with the goal of improving the overall efficiencyof heat recovery from a wellbore throughout various seasons without theneed of supplemental heat from other sources.

SUMMARY OF THE INVENTION

According to one aspect of the invention there is provided a groundsource including rock material, the method comprising:

providing a geothermal system comprising (i) an outer pipe supportedwithin a borehole to extend longitudinally and downwardly into theground source from a top end to a bottom end of the outer pipe in whichthe bottom end of the outer pipe is closed, (ii) an inner pipe withinthe outer pipe to extend longitudinally from a top end of the inner pipein proximity to the top end of the outer pipe to a bottom end inproximity to the bottom end of the outer pipe so as to define an innerpassage extending longitudinally through the inner pipe and so as todefine an outer passage extending longitudinally within the outer pipewithin an annular space between the inner pipe and the outer pipe inwhich a bottom end of the inner passage is in open communication with abottom end of the outer passage, and (iii) piping in communicationbetween the top end of the inner pipe and the top end of the outer pipesuch that the inner and outer pipes form a closed loop; operating thegeothermal system in either one of a first mode to store heat in theground source or a second mode to recover heat from the ground source;

in the first mode, pumping the heat exchanger fluid through the closedloop of the inner passage and the outer passage so as to collect heatinto the heat exchanger fluid from the ground source along a lowerportion of the outer pipe that is in proximity to the bottom end of theouter pipe and so as to transfer heat from the heat exchanger fluid tothe ground source along an upper portion of the outer pipe that is inproximity to the top end of the outer pipe; and

in the second mode, pumping a heat exchanger fluid through the closedloop of the inner passage and the outer passage so as to collect heat inthe heat exchanger fluid from the ground source along at least a part ofthe outer pipe and so as to extract heat from the heat exchanger fluidat the piping.

According to a second aspect of the present invention there is provideda geothermal system for extracting heat from a ground source, the systemcomprising:

an outer pipe supported within a borehole to extend longitudinally anddownwardly into the ground source from a top end to a bottom end of theouter pipe in which the bottom end of the outer pipe is closed;

an inner pipe within the outer pipe to extend longitudinally from a topend of the inner pipe in proximity to the top end of the outer pipe to abottom end in proximity to the bottom end of the outer pipe so as todefine an inner passage extending longitudinally through the inner piperand so as to define an outer passage extending longitudinally within theouter pipe within an annular space between the inner pipe and the outerpipe;

a bottom end of the inner passage being in open communication with abottom end of the outer passage;

piping in communication between the top end of the inner pipe and thetop end of the outer pipe such that the inner and outer pipes form aclosed loop;

a pump arranged to circulate a heat exchanger fluid through the closedloop of the inner passage and the outer passage; and

a controller arranged to operate the pump in either one of a first modeto store heat in the ground source or a second mode to recover heat fromthe ground source, such that:

-   -   (i) in the first mode, the heat exchanger fluid is circulated so        as to collect heat into the heat exchanger fluid from the ground        source along a lower portion of the outer pipe that is in        proximity to the bottom end of the outer pipe and so as to        transfer heat from the heat exchanger fluid to the ground source        along an upper portion of the outer pipe that is in proximity to        the top end of the outer pipe; and    -   (ii) in the second mode, the heat exchanger fluid is circulated        so as to collect heat in the heat exchanger fluid from the        ground source along at least a part of the outer pipe and so as        to extract heat from the heat exchanger fluid at the piping.

Preferably the inner pipe is partially to totally insulated along itslength to minimize heat losses from the ascending heat exchanger fluid.

The heat storage mode described herein works together with the heatrecovery mode so that the overall efficiency of heat recovery throughoutthe thermal storage and recovery process is improved. The method canrecover heat at a temperature adequate for space heating and other useswithout the need of supplemental heat from other sources.

The invention uses the previously developed single well coaxial pipeheat exchanger technology. The inventive nature of this Application isto include several equipment and operational modifications tosignificantly increase the amount of recoverable heat, and also increasethe recoverable temperature at surface. These equipment and operationalchanges can also significantly reduce capital and operating costs.

Much of the heat energy from deeper geothermal wells is used for spaceheating during colder months especially in higher latitude countries.Therefore, the heat demand only occurs over the colder season, leavingthe well dormant when heat demand is limited. The concept behind thisinvention is to continue to recover and store the heat during the periodof normal well dormancy, then release the heat for use when needed. Akey to success for this option is the stored heat has to be readilyavailable when needed.

In the second mode, the heat exchanger fluid may be circulated fortransferring heat from the heat exchanger fluid to the ground sourcealong the upper portion of the outer pipe when heat demands at thepiping are low and transferring heat from the ground source to the heatexchanger fluid along the upper portion of the outer pipe when heatdemands at the piping are high. A variable rate pump may be used to varya flow rate of the heat exchanger fluid between a high heat demand and alow heat demand in the second mode.

In the first mode, heat loss from the piping at the surface isminimized, using insulation and/or by diverting flow to minimize thelength of the flow path between the inner and outer pipes.

In the second mode, the heat exchanger fluid may be circulated forcollecting heat in the heat exchanger fluid from the part of the outerpipe that is surrounded by a part of the ground source having a highesttemperature.

The method may further include (i) providing a heat exchanger incommunication with the piping, (ii) circulating the heat exchanger fluidthrough the heat exchanger in the second mode for extracting heat fromthe heat exchanger fluid at the heat exchanger, and (iii) bypassing theheat exchanger in the first mode.

The method may further include expanding the rock material in the groundsource surrounding the outer pipe by transferring heat into the groundsource along the upper portion of the outer pipe for closing fissuresand other permeable conduits in the rock material and for preventingupward migration of formational fluids and gasses in the ground source.

Heat may be transferred from the heat exchanger fluid to the groundsource along an entirety of the outer pipe in the first mode.

The geothermal system may be operated only in the first mode for anentire season, and operated only in the second mode at varying flowrates for an entire season.

The flow rate of the heat exchanger fluid through the closed loop may becontrollably varied so as to maintain a temperature of the heatexchanger fluid at the top of the borehole at a substantially constantset point temperature.

The heat exchanger fluid may be pumped through the closed loop in acommon direction in both the first mode and the second mode, for examplethe heat exchanger fluid may be pumped downwardly through the outerpassage and upwardly through the inner passage in both the first modeand the second mode.

An inner surface of the inner pipe may be smoother than an outer surfaceof the inner pipe.

A lower portion of the inner pipe may be uninsulated, or alternativelyan entirety of the inner pipe may be insulated.

A cross-sectional area of the inner passage is preferably smaller than across-sectional area of the outer passage.

In the first mode, heat may be transferred from a solar heat collectorinto the heat exchanger fluid at the piping.

The method may further include converting hydrocarbon well having a wellcasing in communication with a production zone into the geothermalsystem by using the well casing as the outer pipe, plugging the wellcasing above the production zone to define the bottom end of the outerpipe, and inserting a pipe string into the well casing to define theinner pipe.

BRIEF DESCRIPTION OF THE DRAWINGS

Some embodiments of the invention will now be described in conjunctionwith the accompanying drawings in which:

FIG. 1 is a schematic representation of the geothermal system operatingin a first heat storage mode; and

FIG. 2 is a schematic representation of the geothermal system operatingin a second heat recovery mode.

In the drawings like characters of reference indicate correspondingparts in the different figures.

DETAILED DESCRIPTION

Referring to the accompanying figures there is illustrated a geothermalsystem 10 installed in the ground such that the ground is used as a heatsource and/or heat sink. In the illustrated embodiment, the geothermalsystem is installed within a reclaimed hydrocarbon wellbore in which theexisting well casing of the wellbore defines an outer pipe 12. The wellcasing is plugged at a location above a production zone of the wellboreto define a bottom end of the outer pipe 12 such that the outer pipeextends longitudinally downward from a top end at a surface of theground heat source to the bottom end which is closed. Alternatively, anyborehole formed in the ground and lined with an outer pipe may be used.

An inner pipe 14 is installed within the outer pipe 12 in which theinner pipe has an outer diameter which is less than the inner diameterof the outer pipe so as to define an annular space between the inner andouter pipes extending along the length of the pipes. The inner pipe 14extends longitudinally downward from a top end adjacent to the top endof the outer pipe to a bottom end of the inner pipe which is spacedabove the bottom end of the outer pipe while remaining in closeproximity to the bottom end of the outer pipe relative to the overalllength of the pipes.

The inner pipe 14 defines an inner passage 16 therein which extendslongitudinally through the inner pipe between the top and bottom ends ofthe inner pipe. Similarly, an outer passage 18 is defined within theannular space between the inner and outer pipes to extend longitudinallyalong the full length of the pipes between the top and bottom endsthereof. The inner pipe 14 is typically insulated along the lengththereof so as to provide a heat insulating barrier between the innerpassage 16 and the outer passage 18 along the length thereof.

The bottom end of the inner pipe 14 remains open so as to be in opencommunication with the bottom end of the surrounding outer pipe.

The top ends of both the inner pipe and the outer pipe are closed offother than to communicate with suitable piping 20 interconnected betweenthe top end of the outer passage and the top end of the inner passagesuch that the inner passage, the outer passage, and the pipingcollectively define a closed loop containing heat exchanger fluidtherein which is circulated through the closed loop as described infurther detail below.

The piping 20 includes an inlet portion 22 communicating with the topend of the outer passage 18 for subsequent connection to a T-junctionwhich joins the inlet portion 22 to a first branch portion 24 from whichit receives fluid in a first mode and a second branch portion 26 fromwhich it receives fluid in a second mode. Similarly, an outlet portion28 of the piping is in open communication with the top end of the innerpassage such that the other ends of the first and second branch portion24 and 26 are connected to the outlet portion 28 with another T-junctionsuch that the first and second branch portions are in parallel flowrelative to one another between the inlet portion 22 and the outletportion 28. The outlet portion 28 receives fluid exiting the innerpassage 16. In this manner, the first branch 24 defines a first loop offluid between the outer passage and the inner passage for use in a firstheat storage mode, while the second branch portion 26 defines a secondloop of fluid between the outer passage and the inner passage for use ina second heat recovery or heat extraction mode.

A suitable pump 30 is connected in series with the outlet portion 28 ofthe piping that is oriented for drawing fluid upwardly through the innerpassage of the inner pipe 16 to be pumped into either one of the firstor second branch portions 24 or 26 such that fluid returns through oneof the first or second branch portions 24 or 26 before descendingdownwardly through the outer passage 18 between the inner and outerpipes.

A heat exchanger 32 is connected in series with the second branchportion 26 to receive the heat exchanger fluid of the geothermal systemcirculated there through in the second heat recovery mode and to permitheat ‘q’ to be extracted from the heat exchanger fluid at the heatexchanger for producing useful work such as generating electrical energyor for heating other devices for example during some modes of operation.

One or more first valves 36 are connected in series with the firstbranch portion 24. In the illustrated embodiment, a single first valve36 may be provided in the first branch portion 24 if the first branchportion is short in length.

Optionally, a solar heat collector 38 (shown in broken line in FIG. 1)may be connected in series with the first branch line 24. In thisinstance a pair of first valves 36 are preferably connected in serieswith the first branch line at opposing ends of the first branch line sothat the solar heat collector 38 communicates with the first branch linebetween the two first valves 36. The solar heat collector 38 is adaptedto collect heat and transfer the heat ‘q’ to the heat exchanger fluid.

A set of second valves 34 are similarly connected in series with thesecond branch line 26 at opposing ends of the second branch line 26 toreceive the heat exchanger between the second set of valves 34.

A controller 40 is provided in operative connection to the pump 30 andthe valves 34 and 36 for operating the pump and the valves in variousmodes and according to various demands for heat, as well as varyingamounts of available heat from the ground heat source.

When there is at least some demand for heat to be extracted from theheat exchanger fluid at the heat exchanger 32, the controller operatesthe system in the second mode to recover heat from the ground source. Inthis instance the second valves 34 are opened and the first valves 36are closed such that the heat exchanger fluid is circulated through theheat exchanger 32 in the second branch portion 24 of the piping as it ispumped through the closed loop by the pump 30. In this mode, heat istransferred from the surrounding ground source into the heat exchangerfluid along a lower portion of the outer pipe adjacent to the bottom endthereof. The heat exchanger fluid then carries the heat upwardly alongthe inner passage for subsequent circulation through the heat exchanger32 where heat ‘q’ can be extracted from the heat exchanger fluid.

When there is low demand for heat, excess heat carried by the heatexchanger fluid can be transferred back into the ground heat sourcealong the upper portion of the outer pipe adjacent to the top endthereof in addition to withdrawing heat at the heat exchanger.

Alternatively, when under high demand for heat at the heat exchanger, inwhich more heat is withdrawn from the heat exchanger fluid at the heatexchanger, the heat exchanger fluid may withdraw heat from thesurrounding ground heat source along both the lower and upper portionsof the outer pipe such that heat is withdrawn from the ground sourcealong the full length of the outer pipe 12.

Temperature sensors may be provided in the outer passage adjacent to theouter boundary formed by the outer pipe at the top end thereof tomonitor temperature of the heat exchanger fluid and/or to monitor thetemperature of the ground source at the top end of the outer pipe. Inthis instance, the controller may vary the flow rate of the heatexchanger fluid being circulated by the pump in order to maintain asubstantially constant temperature at the sensor location.

A reduced flow rate allows more heat to be collected from the groundsource at the lower portion of the well which can result in transfer ofheat back into the surrounding ground source along the upper portion ofthe outer pipe at certain times during the operation of the system.Alternatively, an increased flow rate allows less heat to be collectedfrom the ground source at the lower portion.

The rate of heat withdrawal from the heat exchanger fluid at the heatexchanger can be controlled by varying the flow rate of a second heatexchanger fluid that exchanges heat with the primary heat exchangerfluid of the system at the heat exchanger. Withdrawing more heat fromthe primary heat exchanger fluid in response to increased heat demandslowers the temperature of the heat exchanger fluid being returned alongthe outer passage which may then result in heat being withdrawn from thesurrounding ground source along both the lower and upper portions of theouter pipe in order to reach the target temperature for the heatexchanger fluid.

When there is no demand for heat to be withdrawn from the heat exchangerfluid at the heat exchanger 32, the system may be operated in the firstheat storage mode for storing more heat across a greater portion of theground surrounding the outer pipe according to FIG. 1. In this instancethe first valves 36 are opened and the second valves 34 are closed sothat the heat exchanger fluid circulated by the pump passes through thefirst branch portion 24 of the piping so as to bypass the heat exchanger32. In this instance, heat is continued to be transferred into the heatexchanger fluid from the ground heat source along the lower portion ofthe outer pipe; however, the increased temperature of the heat exchangerfluid results in the excess heat being transferred back into the groundheat source along the upper portion of the outer pipe. To store furtherheat in the ground, the optional solar heat collector 38 can be used tocollect solar heat and transfer the heat ‘q’ into the heat exchangerfluid along the first branch portion 24 of the piping.

In some instances, sufficient heat may be collected by the solar heatcollector to create a sufficient temperature differential that heat maybe transferred from the heat exchanger fluid back into the ground heatsource along the lower portion of the wellbore as well. Alternatively,heated water from the solar collectors and/or from a geothermal sourcecan be stored on surface using an insulated storage tank.

As described above, FIGS. 1 and 2 are visual descriptions of the coaxialpipe configurations. The illustrated example uses, but is not limitedto, a pre-existing oil and gas well. These wells typically have a steelproduction pipe installed and cemented to the enclosed rock formationsfor the total length of the pipe. The purpose of the cemented pipe is tostabilize and isolate the adjacent rock material from the borehole. Oncethe borehole ceases hydrocarbon production, the base of the borehole issealed above the previous production zone. This is done by placing apermanent plug then covering it with a thick protective layer of cement.This is to prevent any inflow of fluids or gases in to the cased well.

This steel production casing becomes the outer pipe of the geothermalheat recovery system. A second smaller diameter pipe is inserted insidethe outer pipe. The inner pipe can either be supported at the surfaceusing existing well hangers or supported at the base by a bottom holeassembly that designed as to not impede water flow between the pipes.

The inside surface of the inner pipe should be smooth, whereas theoutside surface of the inner pipe can benefit from having a roughersurface. This rougher exterior will cause more fluid turbulence whichimproves heat transfer. For shallower, lower temperature wells the innertube can be plastic or fibreglass in composition. For deeper wells withhigher temperatures and heavier pipe weight, a steel string is thepreferred option.

The apparatus is designed to utilize available boreholes that are nolonger in use. Repurposing existing boreholes can significantly reducethe capital costs of recovering geothermal energy. Oil and gas wellsnormally have three different sizes of production pipe diameters. Theseare 178, 149 and 118 mm diameter. For each converted well the selectedinner pipe size has to leave sufficient room in the outer pipe for theproper flow conditions as described.

There are two separate, distinct modes of operation. These are referredto as the Heat Storage Mode (FIG. 1) and the Heat Recovery Mode (FIG.2). The Heat Storage Mode commences after the existing well has beensanitized, the equipment installed and system tested. The purpose ofthis initial stage is to develop the heat envelope around the well bore.Unheated fresh water is pumped in to the well via the annular space. Asthe water descends it will start to collect heat from the surroundingrock once it reaches the depth where the surrounding rock is warmer thanthe descending water. When it reaches the well bottom the heated wateris redirected back to surface via the inner pipe. Some heat will be lostas the water ascends but it will be re-absorbed by the descending waterwithin the inner pipe. Once the ascending water with the remainingabsorbed heat reaches the surface, it is redirected back down the wellto complete the circuit. Each cycle of the water circuit will slightlyincrease the temperature of the water and the resultant heat will betransferred in to cooler surrounding rock. This stage, which cancontinue for a period of up to 7 months, effectively transfers the heatfrom the lower hot section to the upper cold section of the well. Astime progresses heat will continue to be pushed into the rock formationcausing the heat envelope to expand laterally.

Shortly before the start of the heating season the valves are opened tothe connecting surface heat exchanger system. This is the start of theHeat Recovery Mode of operation. The surface pump is connected to theheat exchanger by a hot and a cold pipe assembly (FIG. 2). After theheat is recovered in the surface heat exchanger the resultant cooledwater is re-injected through the annulus and transported to the wellbottom to recover the maximum amount of additional heat from the highertemperature zone in the well. As the water ascends any additionalrequired heat will be recovered from the stored heat envelope developedduring the previous Heat Storage Mode. In so doing, the maximum amountof heat is recovered from the hotter, lower zone and the upper zone isonly required for topping up the water temperature if insufficient heatwas recovered from the lower zone. Flow direction in the well can bereversed if deemed more efficient for certain well conditions.

The Invention calls for the use of an electrically powered variablefrequency drive pumps (VFD) for controlling the amount of heat transferboth underground and at the surface. When the heating season starts, thedemand for heat is low. For example, the heating season normally startsin Autumn and may need some heat overnight but not during daylighthours. The VFD pump would operate at a very low flow rate and only asmall amount of heat would be transferred. The remaining heat in thewater would then be returned to the well. As the season progresses thedemand for heat increases hitting peak demand in mid Winter After that,demand starts to decrease again until late Spring. When demand increasesthe VFD pump increases the flow volume in order to accommodate the needfor more heat. As well, as the heat demand increases the exittemperature of the return cooled water drops which creates the demandfor more heat recovery from the well. The preferred source for theincreased heat demand is at the bottom of the well where the heat poolis most sustainable. The goal is to complete the heating season bymaintaining a constant maximum temperature of the casing wall atsurface. The design also calls for excess heat in the upper section sothat colder than normal weather conditions can be accommodated by thevariable speed pump and the extra heat stored “in the Bank”.

Once the heating season is over, the well system is returned to HeatStorage Mode in preparation for the next heating season.

This developed heat envelope has two key benefits. With the envelopeheat recovery occurs over the total length of the well. Without the heatenvelope, a coaxial geothermal well can only recover heat from near thebottom of the borehole. The Invention can provide greater heat recoveryand a higher well head temperature. The second benefit is, as the heatenvelope develops, the surrounding rock expands and seals the natural ordrilling induced fissuring and fracturing. This helps prevent upwardmigration of light hydrocarbons (such as methane) or other harmfulfluids and gases. Fugitive methane gas is a Greenhouse Gas (GHG) and isconsidered a major contributor to climate change.

If possible, the volume of the inner pipe should be less than the volumeof the space between the inner and outer pipe.

The outer pipe is sealed at the base of the borehole. The inner pipe isopen at its base.

The bottom of the inner pipe is placed sufficiently above the base ofthe borehole to prevent erosion of the borehole bottom.

The transport fluid is fresh water, and may have minor amounts ofcorrosion inhibitors, antifreeze and anti-friction materials ifwarranted.

The inner and outer pipes will be connected at surface by a variablespeed pump and two sets of valves. One set of 2 valves control the waterflow from the well to the surface heat exchanger and back. The thirdvalve controls the water flow through the bypass pipe as shown in FIG.1.

During Heat Storage Mode the 2 valves to and from the surface heatexchanger are closed. The valve connecting the pump to the space betweenthe inner and outer pipes via the bypass pipe is opened as shown in FIG.1.

During Heat Storage Mode unheated water is pumped into the borehole viathe outer passage between the inner and outer pipes.

As it descends heat will be transferred to the water once it reaches adepth where the surrounding temperature is higher than the descendingwater temperature. As the water continues to descend the rate of heatrecovery will increase as the temperature differential between the waterand the adjacent rock increases.

When the heated water reaches the base of the borehole it will beredirected back to the surface via the inner passage of the inner pipe.

During this process the transfer of heat stays within the water loop,and progressively redistributes the heat into the cooler upper rock zoneover time.

With each water circuit cycle the well head temperature increasesslightly until the ascending water temperature at surface stabilizes.

Continued pumping will push more heat in to the surrounding rock as theradius of maximum temperature increases away from the borehole.

Upon completion of the Heat Storage Mode, the Heat Recovery Mode starts.The valves connecting the borehole to the surface heat exchanger pipesare opened and the bypass pipe valve closed as shown in FIG. 2. Theheated water is then redirected through the surface heat exchanger.

The resultant cooled water from the surface heat exchanger is returnedto the borehole to recover more heat, and to complete the heat recoverycircuit.

The equipment configuration is designed to work with a summer solarheating assembly. The resultant additional heat can then then beintroduced in to the Heat Storage Mode and used to top up thetemperature within the heat envelope.

The present invention described herein provides a method of operating acoaxial borehole whereby the rock material surrounding the borehole ispre-heated prior to recovering heat at the surface. The apparatustypically comprises two pipes where the smaller pipe is inserted insidethe larger pipe and fluid is pumped downwardly between the two pipes andreturned upwardly through the inner pipe. The inner pipe, comprising ofa smaller area than the area between the inner and outer pipe, canprovide a higher flow velocity in one direction and a slower flowvelocity in the opposite direction.

The inside wall of the inner pipe, consisting of a smooth surface tominimize flow turbulence, can reduce heat loss.

The outside wall of the inner pipe can consist of a rougher surface toenhance turbulence and heat exchange.

The two pipes at surface are connected together by a variable speed pumpin a closed loop water circuit.

The pipe and pump are sealed from the outside environment to improvepump efficiency and eliminate environmental risks.

The borehole pipe system is connected to a two-pipe system at thesurface to transport the recovered heat to a surface heat exchanger.

The fluid flow from the downhole piping system to the surface pipingsystem is controlled by valves on the two connected surface pipes. Thisis to change the configuration from a Heat Storage Mode to a HeatRecovery Mode.

In the Heat Storage Mode, the water pumped into the well collects heatfrom the lower section of the well and is transferred and stored in thesurrounding rock in the upper, cooler section of the well. Heat that isnot transferred in to the surrounding rock is recycled within the welland will eventually be transferred to the surrounding rock oversuccessive water cycles.

At the completion of the Heat Storage Mode, the valves are opened andthe configuration becomes the Heat Recovery Mode whereby the heatedwater is redirected to the user of the heat.

The inner pipe attributes allow for rapid ascent of the water tominimize both heat loss from the upper section of the well. This allowsfor a greater temperature differential of the water and the surroundingrock which increases heat recovery in the higher temperature zone at itsbase.

The larger exterior space and the rougher exterior wall of the innerpipe in allow for slower fluid velocity and higher flow turbulence.These attributes increase heat recovery from the surrounding rock.

The improved thermal conductivity in the lower section of the wellreduces the heat demand as the transporting fluid ascends in to thepreviously developed heat envelope in the upper, cooler section of thewell. Reduced heat demand in the upper section of the well allows for ahigher more sustainable water temperature at the surface.

The higher recovered water temperature (i) reduces the need forupgrading the heat to match users' heating needs, and (ii) allows theinclusion of more wells that are too shallow or too small in diameter tobe considered economically viable without the Invention.

The heat recovered only requires energy in the form of electricity tooperate the pump. The only source of Greenhouse Gas (GHG) emissions islimited to the amount of carbon based electrical power generation.

The heat envelope developed in the surrounding rock causes the heatedrock to expand. The expansion of the rock leads to closing of the cracksand fissures which reduces or eliminates any potential conduits outsidethe pipe. This is beneficial because seepage of hydrocarbon and noxiousfluids and gases into aquifers and to the surface have a harmful effecton the atmosphere and surface waters.

Since various modifications can be made in the invention as herein abovedescribed, and many apparently widely different embodiments of samemade, it is intended that all matter contained in the accompanyingspecification shall be interpreted as illustrative only and not in alimiting sense.

1. A method of extracting heat from a ground source including rockmaterial, the method comprising: providing a geothermal systemcomprising (i) an outer pipe supported within a borehole to extendlongitudinally and downwardly into the ground source from a top end to abottom end of the outer pipe in which the bottom end of the outer pipeis closed, (ii) an inner pipe within the outer pipe to extendlongitudinally from a top end of the inner pipe in proximity to the topend of the outer pipe to a bottom end in proximity to the bottom end ofthe outer pipe so as to define an inner passage extending longitudinallythrough the inner pipe and so as to define an outer passage extendinglongitudinally within the outer pipe within an annular space between theinner pipe and the outer pipe in which a bottom end of the inner passageis in open communication with a bottom end of the outer passage, and(iii) piping in communication between the top end of the inner pipe andthe top end of the outer pipe such that the inner and outer pipes form aclosed loop; operating the geothermal system in either one of a firstmode to store heat in the ground source or a second mode to recover heatfrom the ground source; in the first mode, pumping the heat exchangerfluid through the closed loop of the inner passage and the outer passageso as to collect heat into the heat exchanger fluid from the groundsource along a lower portion of the outer pipe that is in proximity tothe bottom end of the outer pipe and so as to transfer heat from theheat exchanger fluid to the ground source along an upper portion of theouter pipe that is in proximity to the top end of the outer pipe; and inthe second mode, pumping a heat exchanger fluid through the closed loopof the inner passage and the outer passage so as to collect heat in theheat exchanger fluid from the ground source along at least a part of theouter pipe and so as to extract heat from the heat exchanger fluid atthe piping.
 2. The method according to claim 1 further comprising in thesecond mode, transferring heat from the heat exchanger fluid to theground source along the upper portion of the outer pipe when heatdemands at the piping are low and transferring heat from the groundsource to the heat exchanger fluid along the upper portion of the outerpipe when heat demands at the piping are high.
 3. The method accordingto claim 2 including using a variable rate pump to vary a flow rate ofthe heat exchanger fluid between a high heat demand and a low heatdemand in the first mode.
 4. The method according to claim 1 furthercomprising in the first mode, preventing heat loss from the piping. 5.The method according to claim 1 further comprising in the second mode,collecting heat in the heat exchanger fluid from the part of the outerpipe that is surrounded by a part of the ground source having a highesttemperature.
 6. The method according to claim 1 further comprising:providing a heat exchanger in communication with the piping; circulatingthe heat exchanger fluid through the heat exchanger in the second modefor extracting heat from the heat exchanger fluid at the heat exchanger;and bypassing the heat exchanger in the first mode.
 7. The methodaccording to claim 1 including expanding the rock material in the groundsource surrounding the outer pipe by transferring heat into the groundsource along the upper portion of the outer pipe for closing fissuresand other permeable conduits in the rock material and for preventingupward migration of formational fluids and gasses in the ground source.8. The method according to claim 1 including transferring heat from theheat exchanger fluid to the ground source along an entirety of the outerpipe in the first mode.
 9. The method according to claim 1 includingoperating the geothermal system in the first mode for an entire season.10. The method according to claim 1 including operating the geothermalsystem only in the second mode at varying flow rates for an entireseason.
 11. The method according to claim 1 including controllablyvarying a flow rate of the heat exchanger fluid through the closed loopso as to maintain a temperature of the heat exchanger fluid at the topof the borehole at a substantially constant set point temperature. 12.The method according to claim 1 including pumping the heat exchangerfluid through the closed loop in a common direction in both the firstmode and the second mode.
 13. The method according to claim 1 includingpumping the heat exchanger fluid downwardly through the outer passageand upwardly through the inner passage in both the first mode and thesecond mode.
 14. The method according to claim 13 wherein an innersurface of the inner pipe is smoother than an outer surface of the innerpipe.
 15. The method according to claim 1 wherein a lower portion of theinner pipe is uninsulated.
 16. The method according to claim 1 whereinan entirety of the inner pipe is insulated.
 17. The method according toclaim 1 wherein a cross-sectional area of the inner passage is smallerthan a cross-sectional area of the outer passage.
 18. The methodaccording to claim 1 including transferring heat from a solar heatcollector to the heat exchanger fluid at the piping in the first mode.19. The method according to claim 1 including converting hydrocarbonwell having a well casing in communication with a production zone intothe geothermal system by using the well casing as the outer pipe,plugging the well casing above the production zone to define the bottomend of the outer pipe, and inserting a pipe string into the well casingto define the inner pipe.
 20. A geothermal system for extracting heatfrom a ground source including rock material, the system comprising: anouter pipe supported within a borehole to extend longitudinally anddownwardly into the ground source from a top end to a bottom end of theouter pipe in which the bottom end of the outer pipe is closed; an innerpipe within the outer pipe to extend longitudinally from a top end ofthe inner pipe in proximity to the top end of the outer pipe to a bottomend in proximity to the bottom end of the outer pipe so as to define aninner passage extending longitudinally through the inner piper and so asto define an outer passage extending longitudinally within the outerpipe within an annular space between the inner pipe and the outer pipe;a bottom end of the inner passage being in open communication with abottom end of the outer passage; piping in communication between the topend of the inner pipe and the top end of the outer pipe such that theinner and outer pipes form a closed loop; a pump arranged to circulate aheat exchanger fluid through the closed loop of the inner passage andthe outer passage; and a controller arranged to operate the pump ineither one of a first mode to store heat in the ground source or asecond mode to recover heat from the ground source, such that: (i) inthe first mode, the heat exchanger fluid is circulated so as to collectheat into the heat exchanger fluid from the ground source along a lowerportion of the outer pipe that is in proximity to the bottom end of theouter pipe and so as to transfer heat from the heat exchanger fluid tothe ground source along an upper portion of the outer pipe that is inproximity to the top end of the outer pipe; and (ii) in the second mode,the heat exchanger fluid is circulated so as to collect heat in the heatexchanger fluid from the ground source along at least part of the outerpipe and so as to extract heat from the heat exchanger fluid at thepiping.